In recent years, high-power optical amplifiers (fiber lasers, fiber amplifiers) using a rare-earth doped optical fiber have received attention. Among these, since high-power fiber lasers have the characteristics of being easily cooled and offering high beam quality compared to solid-state lasers, they are attracting a great amount of attention particularly in the field of laser machining. In a high-power fiber laser, a pumping light and a signal light propagate through a fiber, and the signal light is amplified by a stimulated emission process due to the population inversion of rare-earth ions that are pumped by the pumping light. This high-power fiber laser has an amplifying fiber that amplifies the signal light and a light-guiding fiber that guides the amplified signal light to a predetermined place. However, as the output power of fiber lasers has increased, two problems have become evident.
The first problem is stimulated Raman scattering. When stimulated Raman scattering occurs, a part of the output of the signal light is converted into a light (a Stokes light) having a longer wavelength, thus the output power of the signal light falls. This is a problem in both the amplifying fiber and the light-guiding fiber.
One more problem is the amplified spontaneous emission. In the amplifying fiber, in addition to the stimulated emission that amplifies the signal light, spontaneous emission is generated from the rare-earth ions. When this spontaneous emission propagates through the fiber together with the signal light, the spontaneous emission is amplified by the pumping light, leading to a drop in the pumping light that is used for amplification of the signal light, and as a result the output power of the signal light decreases. This is a problem in the amplifying fiber.
In order to rectify these problems, two measures have been proposed. The first measure uses a fiber whose cut-off wavelength of the fundamental mode exists at a wavelength that is longer than the wavelength of the signal light. Thereby, it is possible to suppress propagation of light of longer wavelengths than the signal light, suppress propagation of Raman scattering light and light on the longer wavelength side, and reduce stimulated Raman scattering light and amplified spontaneous emission at longer wavelengths (see “Suppression of Raman Gain in Single-Transverse-Mode Dual-Hole Assisted Fiber,” Optics Express, 13, pp. 8921, 2005). The other measure is a method that involves disposing a ring around a core so as to couple Raman scattering light and spontaneous emission that are guided through the core to a mode that is guided through the ring and cause radiation (see “Suppression of Stimulated Raman Scattering in a Cladding Pumped Amplifier with an Yb-Doped Fiber,” CLEO2006).
However, there are the following problems in the conventional art.
In the fiber that is disclosed in the Suppression of Raman Gain publication, since the cut-off wavelength of the fundamental mode exists at a wavelength that is longer than the wavelength of the signal light, it is not possible to suppress spontaneous emission on the shorter wavelength side than the wavelength of the signal light.
In the fiber that is disclosed in the Suppression of Stimulated Raman Scattering publication, since the coupling to the mode guided through the ring exists at many wavelengths, even the signal light may be suppressed due to manufacturing variations, so that manufacturing is difficult or the tolerance with respect to manufacturing variations is low. Also, since the coupling to the mode that is guided through the ring exists at a comparatively narrow band of approximately 10 nm to 30 nm, it is difficult to accurately match the band to the wavelengths that need to be suppressed in terms of manufacturing, and a band of Raman scattering cannot be entirely covered, and suppression of the spontaneous emission can only be performed in a restricted band.
Exemplary embodiments of the present invention were devised in view of the above circumstances and have an objective to provide an optical fiber that is capable of effectively suppressing propagation of light on either shorter wavelengths or longer wavelengths other than the signal light, and that can be manufactured so that there is no suppression within the wavelength of the signal light without a wavelength in the propagation region at which loss increases rapidly.